Refractory materials for use in equipment for containing and handling molten metals (such components include tundish pouring nozzles and foundry crucibles) have been constructed hitherto using materials which are capable of withstanding severe thermal shock, but which are mechanically weak and have poor wear resistance. Until recently, no ceramic material had been developed which has good thermal shock resistance, good mechnical strength and good wear resistance. The conventional pouring nozzle and foundry crucible, therefore, has been made from a refractory material which is weak, porous, and with a microstructure comprisng coarse grains.
Such conventional refractory materials, as already noted, have good thermal shock resistance. However, their resistance to chemical attack by slag and their resistance to wear by rapidly flowing liquid metal is poor because of the porous, weak nature of the coarse-grained microstructure. In addition, the traditional refractory materials have been impure materials with low melting silaceous (glassy) phases in the grain boundary. The impurities in the materials also contribute to the poor chemical and wear resistance of the traditional refractory materials.
At present, the best commercially available tundish pouring nozzles and foundry crucibles are made of a refractory grade of partially stabilised zirconia (PSZ). This material is expensive and although it has good thermal and mechanical properties, it is not a particularly suitable refractory material for these purposes. For example, the quality of pouring nozzles made with PSZ is very variable. Small amounts of impurities greatly affect the phases formed during manufacture and this has a strong influence on the performance of the product. Glassy material in the grain boundaries softens at the working temperature of the nozzles, and this accelerates erosion of the bore of the nozzles. If any slag is present, the PSZ material is destabilised, such results in even more rapid deterioration of the nozzle.
In a paper entitled "Improved Thermal Shock Resistant Refractories from Plasma-dissociated Zircon", which was published in the Journal of Materials Science, volume 14, l pp. 817-822, 1979, R C Garvie describes a ceramic material formed from dissociated zircon, and containing particles of zirconia. To form the dissociated zircon, grains of zircon sand were dropped through a plasma furnace. The product of this operation--known as dissociated zircon, often abbreviated to "DZ"--comprises spheres of reactive silica in each of which are embedded crystals of zirconia. The silica spheres are about 200 micrometers in diameter; the zirconia crystals have a diameter of about 0.2 micrometer. To make the ceramic material, Garvie mixed a quantity of milled dissociated zircon with about 10 wt percent of zirconia particles having a means particle diameter of 13 micrometers. The powder batch was then moulded into the desired shape and reaction sintered to produce a fine-grained, strong, dense product which exhibited very good thermal-shock resistance. Examination of this material under a microscope showed that it comprised a matrix phase of zircon grains with a dispersed phase of zirconia particles in the grain boundaries.
Tundish pouring nozzles made from this zircon/zirconia material were tested with similar nozzles made from refractory grade PSZ. The testing was carried out under normal working conditions. A tundish pouring nozzle made of the DZ-zirconia composite was still in use after 37 heats in a small tundish used to contain stainless steel. Refractory grade PSZ nozzles could be used for 1 to 4 heats. In a second trial in a large steel mill, one nozzle of each type was used for one heat. The DZ-zirconia composite nozzle bore showed less wear than the bore of the refractory grade PSZ nozzle at the end of the test.
Although the DZ-zirconia composite materials represent a considerable improvement in the development of an inexpensive, high-performance material to replace refractory grade PSZ, such composites have serious disadvantages. For example the cost of the dissociated zircon/zirconia powder batch, although cheaper than PSZ, is still quite high at $A2.70 per kg. In addition, DZ sinters to a high density during firing, but this is accompanied by a linear shrinkage of about 20% or more. With such a high value of linear shrinkage, dimensional control of the fired product is very difficult and it is necessary to fire the ware at a relatively slow rate of heating to prevent cracking. Also, the DZ-zirconia powder batch cannot tolerate the usual amounts of water present in ceramic processing when the powder is subjected to spray-drying or when a water-soluble wax binder is mixed with the powder. When the powder containing such residual water is moulded by die-pressing in a steel die, the pressed material sticks tenaciously to the die wall, thereby generating such powerful frictional forces that the removal of the green were from the die without fracturing the ware is difficult. It has also been found that the DZ-zirconia powder cannot be moulded by slip-casting, using an aqueous slip, because deflocculation of the slip is very difficult.